Communication systems are designed to reliably transfer information using the underlying physical medium. Well-known communication systems like Ethernet use special wiring (e.g., Cat 5 cable) for exchanging information. Such systems, by design, allow all connected stations to exchange data at a fixed data rate. With the increasing need for ubiquitous exchange of information, a new class of no-new-wire systems has emerged. Such systems use existing infrastructure to exchange information. Power line communication systems are one example of such systems.
Power line communication systems use existing AC wiring to exchange information. Owing to their being designed for much lower frequency transmissions, AC wiring provides varying channel characteristics at the higher frequencies used for data transmission (e.g., depending on the wiring used and the actual layout). To maximize the data rate between various links, stations need to adjust their transmission parameters dynamically. This process is called channel adaptation. Channel adaptation results in adaptation information specifying a set of transmission parameters that can be used on each link. Adaptation information includes such parameters as the frequencies used, their modulation, and the forward error correction (FEC) used. In high-speed power line communication systems, good channel adaptation is critical to providing high data rates on all links.
Higher data rates can be achieved in power line communication systems by taking into account the fact that the noise and/or the frequency response of the power line channel between any pair of stations depends on the AC line cycle phase. Power line communication systems share the power line medium with various appliances that draw electric power from the power supply grid. These devices are one of the major sources of noise that affect the characteristics of power line channels. Several types of such devices generate noise that varies with the AC line cycle phase. For example, in some cases the noise around the zero crossing on the AC line cycle is lower by comparison to the noise at the peaks of the AC cycle. Devices like triac-controlled dimmers turn on and off during each AC line cycle. These not only generate impulse noise, but also change the frequency response of the power line channel (e.g., by a load on the line that attenuates some frequencies more than others). Further, several devices that use AC motors (e.g., vacuum cleaners, drills, etc.) generate noise that is also a function of the phase of the line cycle. The net effect is a time varying channel whose noise characteristics and frequency response depend on the AC line cycle phase. Various techniques can be used to take into account the varying channel characteristics. For example, for communication between a pair of stations over an AC power line (or other medium having a periodically varying channel) a different channel adaptation can be assigned to each of multiple phase regions of the periodically varying channel.